Frequency synthesizers are well known in the art and are for the most part formed from an oscillator whose frequency is controlled by a voltage representing the phase difference between a periodic reference signal and a periodic signal obtained by division, by means of a divider, of the frequency of the signal supplied by the oscillator.
At balance, the frequency of the oscillator is equal to the reference frequency multiplied by the division ratio.
An example of a synthesizer of this type is described in French Pat. No. 1 473 202. With the synthesizer described, when the division ratio is changed, a set of discrete frequencies can be obtained whose accuracy and stability are related to the accuracy or the stability of the frequency of the reference signal. The set of frequencies obtained is between two frequency values which depend on the programming possibilities of the divider and on the maximum possible frequency deviation of the oscillator.
A problem arises however when the oscillator frequency must vary in an extensive frequency band, for, in this case, the variation of the division ratio makes the phase dependency of the divided frequency on the reference frequency very difficult to stabilize and, furthermore, the maximum frequency deviation of the oscillator is not always sufficient to allow the desired frequency variation.
To correct the phase dependency instability caused by the increase of the division, one solution consists in introducing into the phase control loop a digital-analog converter coupled as an attenuator of the control voltage of the oscillator, and controlled digitally to reduce the variations of the gain of the control loop due to too high a variation of the rank of the divider. However, the analog solution adopted introduces noise in the control input of the oscillator which makes use thereof delicate and requires additional adjustment circuits for cancelling out the frequency shift caused by the noise voltages delivered by the digital-analog converter.
To obtain a frequency variation of the synthesizer compatible with the frequency deviation possibilities of the oscillator, a solution is also known which is also described in U.S. Pat. No. 1 473 202 and which consists in placing in series with a variable rank divider a divider having several fixed stages, identical stage by stage, i.e. having the same division capacity and to divide, by means of a divider having several stages also fixed, the reference frequency before application thereof to the input of the phase comparator. The use of this solution is complicated for it requires the use of double switches for introducing into the two previously formed division chains the different division stages and an oscillator which is capable of being tuned in a frequency band extending at least over an octave.
The above-mentioned difficulties may be partially resolved by constructing a heterodyning frequency synthesizer using an oscillator with small frequency deviation and a frequency mixer allowing a signal to be obtained, from a mixer signal, whose frequency represents the difference between the frequencies of the mixer signal and the frequency supplied by the oscillator. The intermodulation products are eliminated by a low-pass filter which delivers the desired output signal.
The accuracy and stability of the mixer signal are of the same order of size as those of the reference signal, moreover, these two signals may be obtained from the same source, e.g. by different divisions of the frequency of a signal supplied by a high frequency generator.
Though this device is less delicate and offers better performances than the one where an oscillator is used tunable over a wide band, it remains nevertheless complicated, particularly because of the need to use a high performance low-pass filter at the output of the mixer. Furthermore, the use of a mixer results in deterioration of the phase noise and in the case where the output signal of the synthesizer is used to synchronize a digital data transmission, the phase noise may cause a synchronization error which is not constant in the frequency range obtained.